1. Field of the Invention
The present invention relates to a dispersion compensator, a dispersion compensator system, a method for manufacturing the dispersion compensator, and a method for compensating wavelength dispersion in an optical transmission path.
2. Discussion of the Background
In general, an optical transmission path through which optical communication (optical transmission) is carried out has a positive wavelength dispersion in an optical transmission band. Conventionally, in order to suppress a signal light distortion due to the wavelength dispersion, a dispersion compensation optical fiber including a negative wavelength dispersion in the optical transmission band is connected to the optical transmission path, to thereby compensate the wavelength dispersion in the optical transmission path.
The dispersion compensation technology described above has proceeded toward practical use in an optical transmission having a 10 Gbit/s or less of a transmission speed of a signal light. However, while having tried to have 40 Gbit/s or more of the transmission speed of the signal light for the future use, more precise dispersion compensation is required to respond to 40 Gbit/s or more of the transmission speed. Further, more reliable optical transmission can be achieved if more precise dispersion compensation technology is applied to communication having the transmission speed of at most 10 Gbit/s.
In the dispersion compensation optical fiber described above, an amount of the dispersion compensation is adjusted by changing the length of the optical fiber. Accordingly, since it is difficult to conduct an accurate dispersion compensation less than several tens psec/nm, the dispersion compensation in the high speed transmission described above was difficult. Also, in order to conduct the dispersion compensation of the optical transmission path by the dispersion compensation optical fiber, dispersion compensation optical fibers having lengths corresponding to the dispersion of respective optical paths need to be formed in correspondence to the dispersion of the respective optical paths. Thus, in view of the cost, it is not preferable.
Therefore, considering the above problems, in recent years, there has been studied a variable dispersion compensator in which a dispersion value can be changed.
As an example of the variable dispersion compensator, a Lattice type filter 24 as shown in FIG. 16 has been proposed. The variable dispersion compensator was proposed by K. Takiguchi et al in 1996, and this variable dispersion compensator include a combination of a plurality of Mach-Zehnder interferometers 30.
In FIG. 16, a plurality of Mach-Zehnder interferometers 30 are connected in series, and phase sections of the respective Mach-Zehnder interferometers are provided with thermooptic phase shifters 31. Also, each connecting section of adjacent Mach-Zehnder interferometers 30 is provided with a variable coupler 33. In this variable dispersion compensator, the wavelength dispersion can be changed in a range of +786 psec/nm to −681 psec/nm.
Also, in 2000, F. Horst et al proposed a variable dispersion compensator combined with a ring resonator. In this variable dispersion compensator, the wavelength dispersion can be changed in a range of +1350 psec/nm to −3430 psec/nm.
In the variable dispersion compensator combined with the ring resonator, however, the free spectral range (FSR) becomes small, and the effect of the filter characteristic is given to the wavelength other than the wavelength which needs to be compensated.
Therefore, in order to solve the problem in the free spectral range, K. Takiguchi et al proposed a variable dispersion compensator as shown in FIG. 17 in 2000. In this variable dispersion compensator, as shown in FIG. 17, the Lattice type filter 24 shown in FIG. 16 is connected to an arrayed waveguide grating 11. K. Takiguchi et al stated that the variable dispersion compensator shown in FIG. 17 can eliminate the effect of the free spectral range.
Incidentally, the arrayed waveguide grating 11 is structured by forming a waveguide forming region 10 on a substrate 1, and includes a waveguide structure shown in FIG. 17, for example.
The waveguide structure of the arrayed waveguide grating 11 includes optical input waveguides 2, a first slab waveguide 3 connected to an output side of the optical input waveguides 2, an arrayed waveguide 4 connected to an output side of the first slab waveguide 3, a second slab waveguide 5 connected to an output side of the arrayed waveguide 4, and a plurality of optical output waveguides 6 connected to an output side of the second slab waveguide 5.
The arrayed waveguide 4 propagates a light that has been led from the first slab waveguide 3, and is formed by arranging a plurality of channel waveguides (4a) side by side. Lengths of the adjacent channel waveguides (4a) vary from each other by a predetermined length (AL).
Incidentally, a large number, normally 100, of the channel waveguides (4a) forming the arrayed waveguide 4 are provided. However, in FIG. 17, the number of the channel waveguides (4a) is schematically depicted to simplify the drawing.
In the variable dispersion compensator shown in FIG. 17, output ends of the optical output waveguide 6 of the arrayed waveguide grating 11 are connected to an optical fiber array 21, and a plurality of optical fibers 22 connected to the fiber array 21 are connected to the lattice type filter 24 through an optical fiber array 23.
One end of each optical fiber 22 is connected to the corresponding optical output waveguide 6 in the arrayed waveguide grating 11, and the other end of each optical fiber 22 is connected to the corresponding optical input waveguide 9 in the Lattice type filter 24.
However, the variable dispersion compensator shown in FIG. 17 has a cumbersome structure, resulting in increasing the cost thereof. Also, since only lights having the frequency interval of the arrayed waveguide grating 11 can be multiplexed in this dispersion compensator, a degree of freedom in the dispersion compensation is low.